1. Field of the Invention
The present invention relates to a radar apparatus equipped with a plurality of antennas each switchable between transmission and reception and, more particularly, to a radar apparatus that can accurately and quickly detect an antenna fault during normal operation, can easily make adjustments for changes in the receiving characteristics of each antenna, and can even make corrections, whenever necessary, for changes in characteristics occurring due to changes in ambient temperature.
2. Description of the Related Art
Traditionally, various kinds of radar apparatuses have been used to detect the azimuth, range, and velocity of a target. In recent years, radar apparatuses have come to be mounted on vehicles and used for such applications as collision warning, collision avoidance, automatic cruise control, and automatic driving. Such a radar is used to detect the azimuth, range, and velocity relative to a vehicle traveling ahead on the road.
One type of radar apparatus that can detect range as well as velocity is the frequency-modulated continuous-wave (FM-CW) radar apparatus. In this radar apparatus, a radiowave is transmitted from a transmitting antenna, and a reflected wave returned from a target is received by a plurality of receiving antennas. As the plurality of receiving antennas are spatially separated from each other, the phase of the reflected wave from the same target differs between the receiving antennas. The azimuth to the target can be detected by detecting this phase difference.
Generally, triangular-wave frequency modulation is used for frequency modulation in the FM-CW radar apparatus. The triangular-wave frequency modulation refers to frequency modulation in which the modulating waveform alternates cyclically between a section where the frequency linearly increases and a section where the frequency linearly decreases. The range and velocity relative to the target are computed from the beat frequency in the modulating frequency increasing section (upsweep section) and the beat frequency in the modulating frequency decreasing section (downsweep section). Here, the azimuth to the target can be obtained by scanning an antenna beam narrowed to a prescribed width.
Generally, the methods for beam scanning can be broadly classified into two methods: mechanical scanning and electronic scanning. Digital beam forming (DBF) is one example of the electronic scanning method. The DBF scanning method uses an array antenna comprising a plurality of antennas as the receiving antennas. Antenna beam scanning is performed by using a DBF combining technique which can form an antenna beam in a desired direction by applying a phase-difference, through digital signal processing, to the beat signal obtained for each antenna and combining the results.
According to the DBF scanning method, there is no need to rotate the antennas as with the mechanical scanning method and, hence, the provision of a driving mechanism for rotating the antennas can be eliminated, thus offering the advantages of being resistant to vibration and being able to achieve size and weight reductions. Making use of these advantages, the development of radar apparatuses for automotive applications has been proceeding.
There has also been proposed a DBF radar apparatus that employs an array antenna comprising a plurality of transmit/receive common antennas, but not the array antenna configuration comprising a combination of a transmit-only antenna and a plurality of receiving antennas. This radar apparatus is constructed to radiate a transmit wave from a selected one of the antennas and receive the reflected waves by the other antennas, and provides more channels than there are antennas by sequentially switching from one antenna to another for transmission of the transmit wave. This enhances the directivity of the scanning beam in the DBF scanning method.
However, in the case of a DBF radar apparatus or the like that has a plurality of receiving antennas and performs azimuth detection by using phase information, phase shifts and variations in antenna gain among the antennas can occur due to performance differences among the plurality of antennas. As one approach to addressing this problem, it is practiced to eliminate performance variations by constructing the array antenna by selecting antennas having identical characteristics at the time of fabrication of the radar apparatus. However, this approach is costly as a measure to improve product quality.
Therefore, in another approach, if there exist performance differences among the plurality of antennas, it is practiced to adjust and correct phase shifts and antenna gain variations among the respective antenna channels before shipment from the factory. To correct the phase shift for each antenna channel, use is made, for example, of a reference signal generator, and a signal generated by the generator is transmitted from an adjusting antenna and received by each antenna element; then, using the result, phase correction is applied.
Further, in any radar apparatus that uses a plurality of antennas, a phase shift can occur on each antenna channel due to the deterioration of the antenna element over time, variations in ambient temperature, etc. If azimuth detection is performed without correcting such phase shift, detrimental effects will result, such as a disruption of the scanning direction profile of the combined result or an increase in sidelobe level, causing a degradation of the radar apparatus performance. Therefore, in a radar apparatus that detects azimuth based on phase, such phase shift must be corrected.
However, when using the above correcting means for the phase correction, the reference signal generator and the adjusting antenna must be provided in addition to the radar apparatus itself and, if these components are incorporated into the radar apparatus, not only the size but also the cost of the apparatus increases.
On the other hand, if these components are not incorporated, the phase correction can only be performed, for example, at the time of maintenance, because the above correcting means uses the principle such that the reference signal transmitted from the adjusting antenna is received directly by the receiving antenna and the phase shift is detected based on the received signal. Therefore, this has the problem that the phase correction cannot be performed during normal use of the radar apparatus.
Accordingly, it is an object of the present invention to provide a radar apparatus that eliminates the need for special correction equipment and can accurately and quickly judge, during normal operation, any change existing or occurring in the characteristics of each antenna and apply a correction in accordance with the result of the judgment, and that can be easily be adjusted in an initial setup at the factory before shipment and can even make corrections as needed for temperature variations due to environmental changes during operation, thereby ensuring high accuracy at all times.